1. Field of the Invention
The present invention relates to a spectroscopic method for measuring the concentration changes of analytes (e.g. glucose) in human body tissues (e.g. blood) using a non-invasive technique which does not require taking a sample from the body for examination. It includes a method and instrument for detecting the infrared radiation naturally emitted by the human body through the use of an infrared detector with a combination of adequate filters such as a negative correlation filter or narrow band filters or other detector-filter assemblies.
The method and instrument is based on the discovery that natural infrared emission from the human body, especially from the tympanic membrane, is modulated by the state of the emitting tissue. Spectral emissivity of human infrared radiation from the tympanic membrane consists of spectral information of the blood analyte. This can be directly correlated with the blood analyte concentration, for example, the glucose concentration.
2. Related Art
The current state of the art in measuring sugar levels in body liquids or foods, fruits and other agricultural products requires taking a sample from the object during the examination process. Special instruments are available for determining blood glucose levels in people with diabetes. The technology uses a small blood sample obtained from a finger prick. The blood is placed on chemically prepared strips and inserted into a portable instrument which analyzes it and provides a blood glucose level measurement. Diabetics must prick their fingers to draw blood for monitoring their glucose levels and some of them must do this many times a day.
To eliminate the pain of drawing blood, as well as to eliminate a source of potential infection, non-invasive optical methods for measuring sugar in blood were invented and use absorption, transmission, reflection or luminescence methods for spectroscopically analyzing blood glucose concentrations.
In U.S. Pat. Nos. 3,958,560 and 4,014,321 to W. F. March, a unique glucose sensor to determine the glucose level in patients is described. The patient's eye is automatically scanned using a dual source of polarized radiation, each transmitting in different wavelengths at one side of the patient's cornea. A sensor located at the other side of the cornea detects the optical rotation of the radiation that passed through the cornea. Because the level of glucose in the bloodstream of the patient is a function (not a simple one) of the glucose level in the cornea, rotation of polarization can determine the level of glucose concentration.
In U.S. Pat. No. 3,963,019 to R. S. Quandt there are described a method and apparatus for detecting changes in body chemistry, for example, in glycinemia, where a beam of light is projected into and through the aqueous humor of the patient's eye. An analyzer positioned to detect the beam on its exit from the patient's eye compares the effect the aqueous humor has on said beam against a norm. The change in the glucose concentration is indicated and detected.
In U.S. Pat. No. 4,882,492 to K. J. Schlager there is described a non-invasive apparatus and related method for measuring the concentration of glucose or other blood analytes. It utilizes both diffuse reflected and transmissive infrared absorption measurements. The apparatus and method utilize non-dispersive correlation spectrometry. Distinguishing the light intensity between the two lights paths, one with a negative correlation filter and the other without one, the apparatus provides a measure proportional to the analyte concentration.
In U.S. Pat. No. 4,883,953 to K. Koashi and H. Yokota there is disclosed a method for measuring the concentration of sugar in liquids by the use of near infrared light. The concentration of sugar in the sample is determined by computing the absorption spectrum of the sugar at various depths in the sample. This is measured by a relatively weak power of infrared light, penetrating close to the surface in a sample, and a relatively strong power of infrared light penetrating relatively deeply in the sample.
In U.S. Pat. No. 5,009,230 to D. P. Hutchinson there is disclosed a device for the non-invasive determination of blood glucose in a patient. This glucose monitor is based on the effect of glucose in a rotating polarized infrared light. More specifically, two orthogonal and equally polarized states of infrared light having minimal absorption are passed through a tissue containing blood, and an accurate determination of change in signal intensity is made due to the angle of rotation of these states. This rotation depends upon the glucose level. The method uses transmission of infrared light through the tissue at a minimum level of absorption of the tissue.
In U.S. Pat. Nos. 5,028,787 and 5,068,536 to R. D. Rosenthal et al. there is disclosed a near-infrared quantitative analysis instrument and method of calibration for non-invasive measures of blood glucose by analyzing near-infrared energy following interactance with venous or arterial blood, or transmission through blood contained in a body part.
In U.S. Pat. No. 5,054,487 to R. H. Clarke there is disclosed a method for non-invasive material analysis, in which a material is illuminated at a plurality of discrete wavelengths. Measurements of the intensity of reflected light at such wavelengths are taken, and an analysis of reflection ratios for various wavelengths are correlated with specific material properties such as concentration of analytes.
Another disclosed method for measuring blood sugar (U.S. Pat. Nos. 5,146,091 and 5,179,951 to Mark B. Knudson) involves testing body fluid constituents by measuring light reflected from the tympanic membrane. The testing light and a reference light at a glucose sensitive wavelength of about 500 to about 4000 wave numbers (cm.sup.-1) are directed toward the tympanic membrane which contains fluid having an unknown concentration of a constituent. A light detector is provided for measuring the intensity of the testing light and the intensity of the reference light, both of which are reflected and spectrally modified by the fluid. A light path distance measurer is provided for measuring the distance of a light path traveled by the testing light and a reference light. A circuit is provided for calculating the level of the constituent in the fluid in response to a reduction in intensity in both the testing light and the reference light and in response to the measured distance.
In U.S. Pat. No. 5,313,941 there is disclosed a noninvasive pulsed infrared spectrophotometer for measuring the concentration of at least one predetermined constituent of a patient's blood. It consists of an infrared source which emits broadband pulses of infrared light including different wavelengths of at least 2.0 micrometer. It consists of an infrared detector which detects light at said wavelengths and has passed through the arterial blood vessel of the patient and has been selectively absorbed by at least one predetermined constituent.
In another disclosed method (U.S. Pat. No. 5,341,805 to M. Stravridi and W. S. Grundfest) a glucose monitor determines the concentration of glucose in a sample by monitoring fluorescent light produced directly by any glucose present in the sample. It illuminates the sample with ultraviolet excitation light which induces glucose to fluoresce. A detector monitors the return light in two wavelength bands. One wavelength band includes a characteristic spectral peak of glucose fluorescence; the other wavelength band is a reference band having known spectral characteristics. A processor is used to determine the concentration of glucose in the sample.
In U.S. Pat. Nos. 5,360,004 and 5,379,764 to D. Purdy et al. there is disclosed a method and apparatus for noninvasive determination of the concentration of at least one analyte in a mammal. A portion of the mammal's body is irradiated with incident near-infrared radiation, where the incident radiation includes two or more distinct bands of continuous-wavelength incident radiation. The resulting radiation emitted from that portion of the body is sensed and a value for the concentration of the analyte is derived.
In U.S. Pat. No. 5,370,114 to J. Y. Wong et al. there is disclosed a noninvasive blood chemistry measurement apparatus for measuring the concentration of selected blood components. This apparatus is comprised of: a source of exposing light in an infrared spectral region and the means for detecting light emitted from molecules in response to exposing light from said source of light. At least two additional detected signals are monitored and processed at wavenumbers suitable for eliminating temperature and pressure effects on the calculated blood glucose levels.
In U.S. Pat. No. 5,383,452 to J. Buchert there is disclosed a method, apparatus and procedure for the non-invasive detection of sugar concentration changes in blood. The instrument measures sugar concentration changes using natural fingerprints of sugar, a rotation of polarization of light emitted from the biological particle chromophores dissolved with sugar in human liquids. The degree of polarization of light emitted from luminescence centers undergoing interaction with an optically active medium such as sugar is proportional to the concentration of sugar in blood.
In other research (by J. S. Maier et al published in Optics Letters, V. 19, No. 24, Dec. 15, 1994, p.2062 and by M. Kohl et al published in Optics Letters, V. 19, No. 24, Dec. 15, 1994, p.2170) it is shown that the difference in the refractive index between the extracellular fluid and the cellular components can be modulated by tissue glucose levels which affect the refractive index of the extracellular fluid. Researchers designed and constructed a frequency-domain near-infrared tissue spectrometer capable of measuring the reduced scattering coefficient of tissue with enough precision to detect changes in glucose levels in the physiological and pathological range.
Other patents for non-invasively analyzing glucose levels in blood based on various spectroscopic, electrochemical and acoustic velocity measurement methods are as follows:
In U.S. Pat. Nos. 4,875,486 and 5,072,732 to U. Rapoport et al. there is disclosed a nuclear magnetic resonance apparatus, where predetermined water and glucose peaks are compared with the measured water and glucose peaks for determining the measured concentration.
In U.S. Pat. No. 5,119,819 to G. H. Thomas et al. there is disclosed acoustic velocity measurements for monitoring the effect of glucose concentration upon the density and adiabatic compressibility of serum.
In U.S. Pat. No. 5,139,023 to T. H. Stanley at al. there is disclosed a method for non-invasive blood glucose monitoring by correlating the amount of glucose which permeates an epithelial membrane, such as skin, with a glucose receiving medium over a specified time period. The glucose receiving medium is then removed and analyzed for the presence of glucose using a conventional analytical technique.
In U.S. Pat. No. 5,140,985 to J. M. Schroeder et al. there is disclosed a measuring instrument and indicating device which gives an indication of blood glucose by metering the glucose content in sweat, or other body fluids, using a plurality of oxygen sensors covered by a semi-porous membrane. The device can be directly attached to the arm; the measuring device will react with localized sweating and indicate the wearer's blood glucose level.
The above described state of the art in non-invasive blood glucose measurement devices contains many approaches and indicates the importance of the problem. None of the described devices have yet been marketed. Some inventors claim that instruments being developed give accurate blood glucose level readings and can be used for home testing by diabetics. These instruments have limitations stemming from the use of near infrared light for measurement of absorption, transmission or reflectance; in this region of the spectrum one can observe interference in absorption from other chemical components. Analyses based on only one or two wavelengths can be inaccurate if there is alcohol in the blood or any other substances that absorb at the same frequencies. In addition, these analyses can be thrown off by instrument errors, outlier samples (samples with spectra that differ from the calibration set) physiological differences between people (skin pigmentation, thickness of the finger). Methods of near infrared spectroscopy must be coupled with sophisticated mathematical and statistical techniques to distinguish between non-glucose sources and to extract a faint glucose spectral signature. Another limitation of these types of blood glucose testers is that they have to be custom calibrated for each user. The need for individual calibration results from the different combination of water levels, fat levels and protein levels in various people which cause changes in the absorption of near infrared light. Since the amount of glucose in the body is less than one thousandth that of other chemicals (and all of them possess absorption in the near infrared), variations of these constituents which exist among people may make universal calibration unlikely.
Other, non-invasive but also non-direct methods and instruments attempt to determine blood glucose content by measuring the glucose in sweat, saliva, urine or tears. These measurements, which can be quite reliable from the chemical analysis point of view, do not determine blood glucose levels because of the complicated, and not always well-defined, relation between blood glucose levels and glucose concentration in other body fluids. Other invented methods like acoustic velocity measurements in blood, are not very reliable because of the lack of well established and simple relations with blood glucose levels.
None of the above described methods and devices for the non-invasive measurement of blood glucose, or other biological constituents of the human body, explore the fact that the human body naturally emits very strong electromagnetic signals in the micrometer wavelength. Non-invasive optical methods already invented for sugar determination use absorption, transmission, reflection, luminescence or scattering methods in near-infrared or infrared spectral regions for spectroscopically analyzing blood glucose concentration. As in standard spectroscopical methods one needs a source of electromagnetic radiation in certain wavelengths and a means of detecting the resulting transmitted, absorbed, luminescence radiation after it undergoes interaction with a measured medium, e.g. blood or other tissue, to determine the concentration of biological constituents of the human body using a number of technical approaches.
Infrared sensing devices have been commercially available for measuring the temperature of the objects. Infrared thermometry is used in industry to remotely measure process and machinery temperatures. In medical applications these methods are used to measure patients temperature without physical contact. One can measure a patient's skin temperature or, more reliably, patient temperature by quantifying the infrared emission from the tympanic membrane. The tympanic membrane is known to be in an excellent position to measure body temperature because it shares the blood supply with the hypothalamus, the center of core body temperature regulation. The tympanic thermometer uses the ear. It is inserted into the auditory canal so as to sufficiently enclose the detector apparatus such that multiple reflections of the radiation from the tympanic membrane transform the auditory canal into a "black body" cavity, a cavity with emissivity theoretically equal to one. In such a way the sensor can get a clear view of the tympanic membrane and its blood vessels to determine the amount of infrared radiation emitted by the patient's tympanic membrane.
Plank's law states a relationship between radiant intensity, spectral distribution and temperature of the blackbody. As the temperature rises, radiation energy is increased. Radiation energy varies depending on wavelengths. The peak value of the radiant emittance distribution shifts to the short wavelengths side with an increase in temperature, and radiation occurs over a wide wavelength band. Total energy radiated from the blackbody and measured by a noncontact infrared thermometer is a result of the total energy emitted over all the wavelengths. It is proportional to an integral of Planck's equation with respect to all wavelengths. It is described in physics by the Stefan-Boltzman law.
A number of U.S. patents describe a different idea and design of tympanic, noncontact thermometers. As an example one can reference: U.S. Pat. No. 4,790,324 to G. J. O'Hara; U.S. Pat. Nos. 4,932,789 and 5,024,533 to Shunji Egawa et al.; U.S. Pat. Nos. 4,797,840 and 5,178,464 to J. Fraden; U.S. Pat. No. 5,159,936 to M. Yelderman et al.; U.S. Pat. No. 5,167,235 to A. R. Seacord et al.; and U.S. Pat. No. 5,169,235 to H. Tominaga et al. In these patents various technical approaches are described concerning stabilization and calibration of such noncontact thermometers. Commercially few such thermometers are available. These include: Thermoscan Instant Thermometer Model No. HM-2 for home use by Thermoscan Inc., 6295 Ferris Square, Suite G, San Diego, Calif. 92121-3248 and other instruments such as Thermoscan PRO-1 and PRO-LT for clinical use.